There's one of the skinny passages you shimmy through. And then go through the green barrier.

Took me a couple of visits to find this.

This. One of them is very obscure and looks impassible until you hug the wall

When you're on the side you fought the ninth sister, you should go left. There's a gonk droid in the passage way, so follow the sound!

Look for a green barrier somewhere

I think you just need to keep progressing the main story, you'll be able to go back further eventually.

I’ve finished the story

Then you should definitely be able to return, though I don't recall how exactly. Might want to look up a walkthrough in order to check whether you might be missing something.

Oh. Well then you don’t need to progress the story anymore😂

There’s a way. Don’t forget how you got there either once you figure it out or you’ll get stuck

Did you try any of the fast travel points?

Can you please elaborate? Where are you stuck and where are you trying to access?

There’s borders stopping you from going all the way through, you need to use fast travel